FIG. 1 is a diagram illustrating examples of an optical communication device 11 that performs communication using a wavelength division multiplexed signal, an optical Add/Drop unit 21, a controller 22, and a node controller 23.
The optical communication device 11 includes a demodulator 12, a clock data recovery unit (CDR) 13, and a deframer (DeFramer) 14. The optical communication device 11 includes an electrical/optical converter (E/O) 15, an optical/electrical converter (O/E) 16, a framer (Framer) 17, a driver 18, an intensity modulator 19, and a transmission optical source 20 that is a variable wavelength optical source.
A client signal of 10 Gbit/s output from a client device that is not illustrated in FIG. 1 is converted into an electrical signal by the optical/electrical converter 16, and is converted into a specific frame format by the framer 17. Information relating to a format converted in the framer 17 and the deframer 14 to be hereinafter described is set by the controller 22. A signal converted in the framer 17 is amplified by the driver 18, and is converted into a wavelength division multiplexed signal by the intensity modulator 19 to be output to the optical Add/Drop unit 21. The modulation of a transmission signal performed in the intensity modulator 19 is carried out on the basis of a signal wavelength output from the transmission optical source 20.
The wavelength division multiplexed signal (WDM signal) output from the optical Add/Drop unit 21 is converted into an electrical signal by the demodulator 12, and a clock is extracted in the clock data recovery unit 13. After that, the extraction of header information and the conversion of a signal frame are performed in the deframer 14. After that, the WDM signal is finally converted into an optical signal in the electrical/optical converter 15, and is output to the client device.
In some cases, a line on which a main signal is transmitted is changed from a currently used line to a protection line for the protection of the line. In such a case, by adding change information into the overhead portion of the signal, a receiving side can recognize the change of the line. When a transmitting side and the receiving side have change protocols and perform handshake operations with each other, it is also desirable to add wavelength change information into the overhead portion of a signal and transmit the signal.
Currently, WDM communication is performed using an intensity modulation method, and receivers can receive wideband wavelengths. When an optical Add/Drop device (OADM) or an optical cross connect device (OXC) is used, signal communication (communication) can be performed only by carrying out the changeover of an optical switch such as OADM or the like.
In a coherent optical communication system, there has been a technique in which a signal corresponding to the channel power of a channel wavelength component that passes through an optical filter and a signal corresponding to an ASE noise light component are detected, and an optical signal-to-noise (SN) ratio is calculated on the basis of the detected signals. An example of documents that disclose such a technique is Japanese Laid-open Patent Application Publication No. 2002-280962. Using the technique, the variation of the optical SN ratio can be reduced.
In the coherent optical communication, in a case in which the wavelength of the transmitting side is changed, if the wavelength of local oscillator light in a coherent optical receiver is not matched to the wavelength of the transmitting side, signal communication may not be performed. Therefore, in synchronization with the change of the wavelength of the transmitting side or the switching of the OADM or the like, it is also desirable to change the wavelength of the local oscillator light on the receiving side.
For example, in order to perform wavelength change, it is desirable to transmit, to the receiving side, information used for giving notice of the change of the wavelength using the overhead portion or payload of the main signal. However, unless the wavelength of the local oscillator light of the receiving side is changed to substantially the same wavelength as that of the transmitting side, the receiving side may not know the change of the wavelength. The receiving side may not recognize whether it is difficult to perform communication, owing to the problem of a transmission path or the like, the signal itself is not input, or it is difficult to perform communication, owing to the change of the wavelength.
In order to change the wavelength of a variable wavelength optical source used for the local oscillator light and synchronize the wavelength of the variable wavelength optical source with the wavelength of an optical signal actually input to the receiver, it is desirable to cause the level and wavelength thereof to converge with the wavelength of a target. Therefore, while it is desirable to adjust the wavelength with feedback, it takes time to adjust the wavelength. If a feedback circuit and a monitoring function are provided for feedback, a circuit size and a cost may be increased.